Thermoplastic starches, either alone or in combination with other polymers, are often used in the manufacture of articles for which water or biological degradation are considered important. The thermoplastic starch is typically formed by plasticizing a native starch with a functional plasticizer or mixture of plasticizers, such as polyfunctional alcohols (e.g., ethylene glycol, propylene glycol, or glycerol). Conventional thermoplastic starches, however, are often problematic in that they absorb moisture and age during storage, exhibit processing problems, and lack the requisite mechanical strength, ductility and toughness for many applications. Various techniques were thus developed in an attempt to improve the properties of thermoplastic starch. U.S. Pat. No. 6,933,335 to Berger, et al., for instance, describes a technique that involves extruding a mixture of a thermoplastic starch and at least one hydrophobic polymer with the addition of a hydrolysis component based on polyvinyl acetate, lower functional alcohols and/or water, and an acidic catalyst (e.g., dibutyl tin oxide). According to Berger, et al., the acidic catalyst enhances the transesterification or crosslinking of the starch, the hydrophobic polymer, and hydrolysis component. For this reason, the starch component of the blend has a molecular weight that is only minimally reduced relative to native starch.
Despite the techniques developed, it has still proven problematic to form melt-extruded substrates (e.g., nonwoven webs, films, etc.) from thermoplastic starches. Films, for example, typically require polymers of appropriate molecular weights and suitable melt viscosity for processing. It is often difficult, however, to achieve both mechanical strength and water/biological degradation from such polymers. As such, a need currently exists for a thermoplastic starch that exhibits good mechanical properties and is capable of water and/or biological degradation.